Have you seen these abnormal growths on plant leaves?

Galls on tree in Longmont, ColoradoPhoto byColorado Martini Publishing LLC / YouTube

Walking on a green belt in Longmont, Colorado, I noticed an odd growth on certain trees. They seem to appear each year and look like normal growth on the leaf. As if they are some sort of seed pouch. According to Colorado State University (CSU), these are “Insect and Mite Galls.”

CSU refers to them as chemical secretions produced during feeding or egg laying. Galls can also form following mechanical injury from insect feeding. The plant forms an abnormal growth on its tissue.

Galls can occur on any part of the plant. But the most common are leaves, stems, and flowers. They are product by insects and mites and rarely cause serious threats to the plant’s health. They usually start in late spring and early summer. This is the time that an adult insect starts laying eggs. Once the gall forms, it remains on the plant for an extended time.

Rhodium gallPhoto byIsidre blanc, CC BY-SA 4.0 via Wikimedia Commons

Once galls formation starts it is usually irreversible. Heavy infestations that occur repeatedly for several seasons may slow the growth of your plant. Refer to an arborist if you are concerned about growths in your landscape. They might often cover the leaves with an oil type spray onto the leaves during the egg-laying cycle.

Here in Colorado, usually a wasp, aphid, or midge lays eggs on the branch or buds of a tree or shrub. When the larva emerges, it secretes chemicals that stimulate the plant to make a gall structure from its own tissue. This acts as protection for the larva. It can also provide food for the larva. At times, by the look of the structure you can determine the kind of insect that created the gall.

The gall can be beautiful and bizarre. These growths that are produced in response to an invading organism can grow in various shapes, sizes, and colors. Depending on the insect species, plant galls can be shaped like stars, baskets, clubs, wigs, or bowl structures. They can be quite spectacular.

Are galls edible?

They are unsuitable for consumption. They are high in tannic acid which makes them unsuitable for humans to eat. In Missouri and Arkansas, a particular species of oak gall falls from the trees in the autumn and is gathered for livestock feed.

Are rose hips on roses galls?

I have often wondered if a rose hip was a gall. The rose hip, also called “rose haw” and “rose hep,” is a fruit of the various species of rose plant. If you have seen one, they are typically red to orange, but can be dark purple to black in some species. Rose hips begin to form after pollination of flowers in spring or early summer and ripen in late summer through autumn. The rose hip is an edible fruit after the blooms fade.

The rose hips are popular additions to breads, pies, jam, jelly, marmalade, syrup, soup, tea, wine, and other beverages. They are very high in Vitamin C. Rose hips can be eaten raw, like berries, if you avoid the hairs inside the fruit. The hairs can be used as itching powder.

References

• Insect and Mite Galls: 05557.pdf (colostate.edu)
• Great Galls! - Colorado Native Plant Society (conps.org)
• Field Guide to Plant Galls in Western US: https://press.princeton.edu/books/paperback/9780691205762/plant-galls-of-the-western-united-states
• Gall - Wikipedia
• Rose hips: Rose hip - Wikipedia
• Rose hip edible: How to Harvest and Use Rose Hips (thespruce.com)
• Plant Galls: TPWD: Plant Galls -- Young Naturalist (texas.gov)